Separate compaction filter for each compaction

Summary:
If we have same compaction filter for each compaction,
application cannot know about the different compaction processes.
Later on, we can put in more details in compaction filter for the
application to consume and use it according to its needs. For e.g. In
the universal compaction, we have a compaction process involving all the
files while others don't involve all the files. Applications may want to
collect some stats only when during full compaction.

Test Plan: run existing unit tests

Reviewers: haobo, dhruba

Reviewed By: dhruba

CC: xinyaohu, leveldb

Differential Revision: https://reviews.facebook.net/D12057

include/leveldb/compaction_filter.h

@@ -41,6 +41,31 @@ class CompactionFilter {
   virtual const char* Name() const = 0;
 };
 
+// Each compaction will create a new CompactionFilter allowing the
+// application to know about different campactions
+class CompactionFilterFactory {
+  public:
+    virtual ~CompactionFilterFactory() { };
+    virtual std::unique_ptr<CompactionFilter> CreateCompactionFilter() = 0;
+
+    // Returns a name that identifies this compaction filter factory.
+    virtual const char* Name() const = 0;
+};
+
+// Default implementaion of CompactionFilterFactory which does not
+// return any filter
+class DefaultCompactionFilterFactory : public CompactionFilterFactory {
+  public:
+    virtual std::unique_ptr<CompactionFilter>
+    CreateCompactionFilter() override {
+      return std::unique_ptr<CompactionFilter>(nullptr);
+    }
+
+    virtual const char* Name() const override {
+      return "DefaultCompactionFilterFactory";
+    }
+};
+
 }  // namespace leveldb
 
 #endif  // STORAGE_LEVELDB_INCLUDE_COMPACTION_FILTER_H_

include/leveldb/options.h

@@ -25,6 +25,7 @@ class Logger;
 class MergeOperator;
 class Snapshot;
 class CompactionFilter;
+class CompactionFilterFactory;
 
 using std::shared_ptr;
 
@@ -84,8 +85,13 @@ struct Options {
   // Default: nullptr
   const MergeOperator* merge_operator;
 
+  // The client must provide compaction_filter_factory if it requires a new
+  // compaction filter to be used for different compaction processes
   // Allows an application to modify/delete a key-value during background
   // compaction.
+  // Ideally, client should specify only one of filter or factory.
+  // compaction_filter takes precedence over compaction_filter_factory if
+  // client specifies both.
   // Default: nullptr
   const CompactionFilter* compaction_filter;
 
@@ -506,6 +512,11 @@ struct Options {
   // Default: a factory that provides a skip-list-based implementation of
   // MemTableRep.
   std::shared_ptr<MemTableRepFactory> memtable_factory;
+
+  // This is a factory that provides compaction filter objects which allow
+  // an application to modify/delete a key-value during background compaction.
+  // Default: a factory that doesn't provide any object
+  std::shared_ptr<CompactionFilterFactory> compaction_filter_factory;
 };
 
 // Options that control read operations